Aims First, to develop an index that quantifies biogeographical patterns based on the basic descriptors of presence/absence distribution patterns (F, frequency; C, connectivity; G, grouping). Second, to test the proposed biogeographical index on a data set of macrophyte communities in the Arabian Sea using distribution data of macro-algae and seagrasses throughout the Indian Ocean.
Location Arabian Sea (regional macrophyte community data) and the larger Indian Ocean (oceanic distribution data).
Methods The proposed index is derived from the Tulloss tripartite similarity index. The tripartite biogeographical index (TBI = √F × C × G) is calculated for a specific taxon and incorporates several fundamental parameters of presence/absence data in grid cell (block) patterns. TBI accounts for the relative abundance of a taxon, the average grouping of its occurrences, the average of minimal absence intervals between taxon presences and the largest coherent cluster of taxon occurrences, and also incorporates dispersal aspects. The macrophytes from the case study are among the best documented marine organisms in the Indian Ocean. The regional distribution data from the Arabian Sea result from exhaustive species lists from Masirah Island (Oman) and the Socotra Archipelago (Yemen).
Results TBI values fit a linear scale corresponding to the proportional presence and the distributional spread of taxa within a given geographical area. The three functions that constitute TBI can be evaluated separately or in association with TBI, to give detailed information on the important factors that characterize the biogeographical distribution of a taxon (or larger entities consisting of multiple taxa such as communities). The case study on Arabian macrophytes clarifies the use and explanatory power of the index.
Main conclusions The biogeographical descriptors can be combined into an index which accurately quantifies taxon occurrences and distribution types on a single linear scale from rare/scattered to abundant/grouped. Together with its three functions, the index allows for a non-arbitrary selection of taxa and taxon groupings based on their distribution pattern. The analysis of TBI values for the Arabian macrophyte communities confirms previous biogeographical findings and enables more detailed statistical analyses of the distribution data.